In the production of hydrocarbons from marine oil and gas deposits, a fluid communication system from the sea floor to the surface is required. Such a system usually includes multiple conduits through which various fluids flow between a subsea well or pipeline to a surface facility. The multiple conduits for communicating with a surface facility typically include subsea trees, manifolds, production and export flowlines, buoys and riser systems.
One method for producing hydrocarbons from marine oil fields is to use a fixed facility attached to the seafloor, however, known fixed facilities can be enormously expensive, and this is especially true for the development of deepwater production facilities. A deepwater discovery prospect may have the potential to justify full field development which would include the development of a dry-tree completion unit, such as a spar. However, evaluation of such a prospect must be carefully managed. Care must be taken so as to minimize the drilling of unnecessary and/or unproductive wells. In deepwater production, drilling costs are becoming so large that the cost of the subsea wells themselves may comprise substantially more than half of the total cost of deepwater development. Further, reservoir performance characteristics cannot be predicted with certainty, this is especially true in frontier provinces where there is little or no previous operating experience in that region.
To offset some of the costs associated with a deepwater discovery prospect, an Early Production System (“EPS”) can be initially used. Using an EPS, an operator can begin to gain field operating knowledge while at the same time generating revenue to amortize investment from the early production of hydrocarbons from the prospect. An EPS can be expected to produce from a wet-tree well system on the seafloor. Should the use of the EPS show that full field development is desirable, it may be technically and economically attractive to preserve the productive capacity of the wet-tree well system through the adaptation to a dry-tree well system or vertical access service. The economics of full field development may require that a drilling or workover rig be positioned on the production platform to reduce field development costs. The drilling or workover rig can be used to drill new wells, to work over existing wells, or to even to maintain submersible pumps.
The advantage of utilizing an EPS is that a limited number of subsea wells can be drilled to delineate a prospective discovery, and these wells can be produced for a time frame on the order of months to years to quantify reservoir performance characteristics. Depending on the prospect, the wells may be widely dispersed and drilled vertically or they may be clustered in a “drill center” and drilled directionally. A drill center has the advantage that the wells can be manifolded and tied back to a host vessel by a pair of flowlines and risers to form a round-trip pigging loop (“pigging loop”). Wet-trees from a subsea drill center are positioned to be compatible with the seafloor well pattern for a dry-tree production unit such as a spar. One example of a wet-tree well pattern is a square with 50 feet of separation between the wells. A drill center can then use the wells positioned along one or more sides of the square for production. Alternatively, the wells can be in clusters that are positioned so that the dry-tree unit can be moved using its mooring system to reach over the wells for working over the old wells, drilling wells, or even for well maintenance such as submersible pump replacement.
It is an object of the present invention to provide a low cost subsea manifold that can be used with an EPS to produce, maintain and/or workover multiple subsea wet-tree wells through a common riser system. It is also a further object of the present invention that the subsea manifold be able to control the fluid from a multiple of subsea wet-tree wells while at the same time giving the operator the option to isolate production from a single subsea well for production evaluation. It is an additionally object of the present invention to control the direction of flow of fluid in the subsea manifold.
It is an objection of the present invention to provide a manifold that allows for efficient pigging of a first flowline or of a first and second flowline.
It is an object of the present invention to enable a wet-tree to be connected to a manifold by a jumper, wherein the manifold is connected to a bottom-founded, top-tensioned riser. The present invention will thereby allow for the production of hydrocarbons, well workover and well maintenance without disconnecting the riser.